Moisture or water vapor is one of the components which can cause an undesired chemical reaction.
For instance, Lithium (used in the production of Lithium batteries) will react violently with water vapor. This is why production of Lithium batteries can only be done in an extremely dry (dehumidified) area.
Explosives and propellants are also known to be sensitive to chemical reactions with moisture. Undesired reactions might lead to an unstable product (danger of explosion) or the forming of unwanted and dangerous chemical compounds (ex: hydrogen gas).